For nearly 120 years, the Marine Biological Laboratory (MBL) has offered the Embryology Course, which has played an important role in the training of American developmental biologists. The course trains pre-and post-doctoral students for research careers in developmental biology in a unique physical and intellectual environment, not duplicated elsewhere in the nation. The Embryology Course consists of a series of formal lectures, informal seminars, extended discussions, and intensive laboratory research experience delivered over a six-week period in the summer. Course students are selected who are committed to teaching and research careers in the field of developmental biology, and are chosen from a consistently talented pool of national and international applicants. Traditionally, a number of students from each year's class later move into positions of leadership in the worldwide developmental biology community. The teaching faculty are senior scientists in the field, some of whom serve as instructors and lead research modules in the course, and others give lectures and lead discussions of individual topics. The course consists of three modules, each of which is about 2 weeks long. Each module is staffed by an average of three-four resident instructors, plus a comparable number of outside lecturers. The themes of the modules change from year to year as important developments in the field occur. The course introduces students to a large number of terrestrial and marine embryos including C. elegans, Drosophila, planarians, ascidians, sea urchins, zebrafish, amphibians, chick and mouse. Techniques include current molecular biological methods, embryonic manipulation, microinjection and micromanipulation, microscopy and computer-assisted imaging technologies, and a number of immunochemical and in situ hybridization approaches. Students are challenged to formulate and test hypotheses in a well-equipped research environment and under the guidance of experienced research faculty and assistants. In support of the course, the MBL provides a unique learning environment, laboratory and support facilities, an animal collection facility, and one of the nation's finest research libraries.